1. Field of the Invention
The present invention relates to an image forming system having an original feeding apparatus, and more particularly, it relates to an image forming system, such as a copying machine, a laser beam printer and the like, having an original feeding apparatus for feeding a sheet original to a predetermined position, such as an image reading station, and for resting the sheet original thereon.
2. Related Background Art
In the past, as shown in FIGS. 10 and 21, original feeding apparatuses were divided into two groups.
FIG. 10 shows an original feeding apparatus of switch-back type wherein a sheet original is fed from a sheet tray 4 to an image reading station (platen 3) from an end B of the glass platen 3 and the sheet original is rested on the platen at a predetermined position. Thereafter, an image reading portion (optical system) of an image forming system is shifted to read the image, and then the sheet original is discharged from the end B of the glass platen and is stacked on the sheet tray again.
FIG. 21 shows an original feeding apparatus of a type wherein, similar to FIG. 10, a sheet original is fed to an image reading station (platen 3) from an end B of the glass plate 3 and the sheet original is rested on the platen at a predetermined position. After the image reading operation, the sheet original is discharged from the end B of the glass platen (switch-back type) or the sheet original is discharged from an opposite end C of the glass platen into a sheet path communicating with the sheet tray (closed-loop type). The closed-loop type or the switch-back type is selected in accordance with a size of the sheet original. In the original feeding apparatus of closed-loop type, after the image reading operation, the sheet tray is positioned horizontally or inclined slightly upward to facilitate the re-stacking of the sheet originals on the sheet tray 4.
In an original-through scanning mode in the conventional image forming system, as shown in FIGS. 10 and 11, after the sheet original P from the sheet tray 4 is rested on the glass platen 3 at any position, an optical system 213 is fixed, and the image reading is effected while discharging the sheet original P in a direction A.
However, there were the following drawbacks in performing the image formation process by the aforementioned conventional techniques.
In the switch-back type shown in FIG. 10, since the next sheet original P was rested on the platen 3 after the first sheet original P had been discharged from the platen, lengthy a time (referred to as "original exchange time" hereinafter) was required to discharge the first sheet original from the platen after the image reading operation and to set the next sheet original to be treated on the platen (corresponding to a time for feeding two sheet originals). In a high speed image forming system, since a copy sheet-to-copy sheet time (for example, a value obtained by dividing a distance between a trailing end of the first sheet and a leading end of the second sheet by a process speed) is shorter as the system is operated at higher speed, when "1 to 1" image formation (one image is formed on one sheet) is effected, if the sheet original exchange time is not smaller than the copy sheet-to-copy sheet time, it is impossible to make the productivity in the "1 to 1" image formation in the image forming system 100%. In the original feeding apparatus of the type shown in FIG. 10, since the sheet original exchange time was long, 100% productivity could not be achieved.
On the other hand, in the original feeding apparatus of the type shown in FIG. 21, although the sheet original can be fed by the switch-back mode as similar to FIG. 10 regarding a large size (for example, A3 size) sheet originals, a half size (for example, A4 size and the like) sheet original is treated by the closed-loop mode. As shown in FIG. 22, in the closed-loop path, after the image reading operation regarding the first sheet original, the next sheet original is rested on the image reading station. In the closed-loop type, since the sheet original exchange time is relatively short (a time required for feeding the sheet original by a distance corresponding to one sheet original plus a gap between two sheet originals), it is possible to exchange the sheet originals faster than the switch-back type shown in FIG. 10; but, if a further high speed operation is desired, since the feeding distance for the sheet original is already fixed or determined, the feeding speed must be increased. However, if the feeding speed is increased, controlling the stop position with high accuracy will become difficult, the potential to damage the sheet original will be increased when the sheet original is jammed, and the system will become larger and more expensive and louder because a large-sized motor must be used for the higher speed operation.
To avoid this, in order to perform the high speed image formation without using the high speed sheet original feeding, it is known to fix the image reading portion of the image forming system and to read the image while shifting the sheet original. When the original-through scanning is effected in the types shown in FIGS. 10 and 21, the image reading is performed while the sheet original is being fed from the end C to the end B of the glass platen.
In the type shown in FIG. 11, when the original-through scanning is effected, after the sheet original is fed onto the glass platen, the sheet original is shifted to the direction A, and meanwhile the image on the sheet original is read.
Further, in the apparatus shown in FIG. 10, a belt conveyor 7 is rotated in a direction E to feed the sheet original P onto the platen 3 and then is rotated in the reverse direction to return the sheet original to a direction D, thereby performing the switch-back feeding. Thus, a motor for driving the belt conveyor is rotated normally and then to the reverse for a short time.
However, in the conventional techniques, since there was the following relation between an original-to-original distance and a copy sheet-to-copy sheet distance in the original-stationary scanning mode and the original-through scanning mode, it was impossible to operate the apparatus at a high speed adequately.
Original-stationary Scanning Mode
When the original-to-original distance is l.sub.1 and the copy sheet-to-copy sheet distance is l.sub.2, l.sub.1 becomes greater than l.sub.2. Thus, in order to coincide the distances with each other, the copy sheet must be stopped for a longer time.
Original-through Scanning Mode
When the original-to-original distance is l.sub.3 and the copy sheet-to-copy sheet distance is l.sub.2, the relation l.sub.1 &gt;l.sub.3 &gt;l.sub.2 is established. Thus, l.sub.3 must be coincided with l.sub.2.
Accordingly, with the conventional arrangement, since the copy sheet-to-copy sheet distance l.sub.2 was constant, it was impossible to operate the apparatus at a high speed adequately.